The present invention relates to a composite overhead stranded conductor obtained by providing an optical fiber cable in an overhead power line formed by stranding a plurality of conductors, or in an overhead ground wire which extends parallel to such an overhead power line.
System protection, control and surveillance of overhead power lines are important for their proper performance. In order to meet these requirements accurately and precisely, composite overhead stranded conductors having optical fibers accommodated in overhead power lines or overhead ground wires are used. The construction of a conventional composite overhead stranded conductor is illustrated in FIG. 1.
In FIG. 1, a spacer 1 is provided in the center of the composite overhead stranded conductors. Spiral grooves 2 are formed in the periphery of the spacer 1, and optical fibers 3 are loosely fitted in the grooves 2. The spacer 1 is accommodated in an aluminum protective tube 4 to form an optical unit. Aluminum-clad steel wires 5 are wound around the optical unit.
Such composite overhead stranded conductors are used in a more hostile environment than that encountered by ordinary optical fiber cables for communications purposes and are required to have better stability in transmission characteristics under such unfavorable conditions.
After installation, the composite overhead stranded conductor is stretched under the effect of its own weight or other tensile stresses caused by temperature elevation due to dielectric currents or abnormal short-circuit currents. As an illustration of the extreme nature of temperature variations, a cable which is normally at about 50.degree. C. is heated up to as high as about 400.degree. C. if a short-circuit current occurs.
It is well known that an optical fiber exposed to high temperatures suffers not only microbending loss due to the shrinkage of the jacket around the fiber, but also absorption loss due to the presence of OH groups. One principal cause for the absorption loss is hydrogen gas which is released from the jacket and diffuses into the fiber core so as to react in defects in the core glass to form OH groups. With the composite overhead stranded conductor, hydrogen gas released from the jacket around the fiber under elevated temperatures is confined within the protective tube. This increases the amount of hydrogen gas that diffuses into the fiber core, thereby causing a greater absorption loss due to OH groups.
These adverse effects of hydrogen gas can be avoided by using an optical fiber whose core or cladding is doped with fluorine. As already mentioned, the primary cause of the increased transmission loss due to hydrogen gas is the absorption loss due to the OH groups formed by reaction with hydrogen gas. Fluorine is capable of preventing the increase in transmission loss by suppressing the formation of unwanted OH groups. However, the fluorine-doped optical fiber has other problems: first, its mechanical strength is reduced, and secondly, microbending is highly likely to occur under varying temperature conditions. Therefore, from a reliability viewpoint, such fluorine-doped optical fiber has only limited use in composite overhead stranded conductors which are subjected to greater temperature variations than ordinary cables and which are typically used in an environment involving relatively large mechanical disturbances such as vibration.